The present invention relates to hydrogen/oxygen fuel cells having a solid-oxide electrolytic layer separating an anode layer from a cathode layer; more particularly, to fuel cell assemblies and systems comprising a plurality of individual fuel cells in a stack wherein air and reformed fuel are supplied to the stack; and most particularly, to such a fuel cell system including an integrated air supply system, including a central air pump, distribution manifold, air control valves, mass air flow sensors, and supply ducts, for controllably supplying air to all required fuel cell system functions.
Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a xe2x80x9csolid oxide fuel cellxe2x80x9d (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two Oxe2x88x922 anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by xe2x80x9creformingxe2x80x9d hydrocarbons such as gasoline in the presence of limited oxygen, the xe2x80x9creformatexe2x80x9d gas includes CO which is converted to CO2 at the anode via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are selectively vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by xe2x80x9cinterconnectxe2x80x9d elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or xe2x80x9ccurrent collectors,xe2x80x9d which may be connected across a load.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. There typically are many gas conduit connections between components in the system. These connections typically are conveying high temperature oxidant gas (air and exhaust) or hydrogen-rich reformate fuel at high temperature. Conventional approaches for conveying these gases include plumbing networks comprising metal tubing, pipes, and fittings. These components often have welded or compression-fitting connections that have the undesirable characteristics of high cost, large size, complexity, and moderate reliability. Typically, each component is directed to a specific function without regard to an overarching system architecture and physical consolidation.
What is needed is a means for reducing the complexity, cost, and size of a solid-oxide fuel cell system by consolidating the auxiliary systems, piping, and connections.
It is a principal object of the present invention to simplify the construction and reduce the cost and size of a solid-oxide fuel cell system.
It is a further object of the invention to increase the reliability and safety of operation of such a fuel cell system.
Briefly described, in a solid-oxide fuel cell system, a compact, highly space-efficient fuel/air manifold assembly conveys high temperature air, exhaust, and hydrogen-rich fuel such as, for example, reformate or pure hydrogen, to and from the core components of the system. The manifold is a three-dimensional assembly of plates and shallow partitioned elements which are easily and inexpensively formed. When assembled, the manifold comprises a network of passageways which allow for the mounting, close-coupling, and integration of critical fuel cell system components. An integrated fuel reformer partially oxidizes liquid hydrocarbon fuel catalytically into hydrogen and carbon monoxide and interacts via heat exchangers to controllably add or subtract heat in various gas flows in the system. An integrated air supply system pressurizes atmospheric air for providing oxygen for the fuel cell reaction, both through and controllably bypassing cathode air heat exchangers; combustion air for a combustor of tail gas from the anodes; cooling air for electronic controls; and reforming air to a liquid fuel vaporizer integral with the reformer.